The invention relates to an apparatus for extruding thermoplastic synthetic plastics material, comprising an extruder housing in which an extruder worm driven by a drive means is bearingly supported for rotation around its axis, said housing being provided with an intake opening for the material to be plasticized, which opening, for example, is connected to a funnel, and with an exit opening for the melt disposed in axial distance from the intake opening and with at least one degasing opening for the escape of gases, and with at least one melt filter through which the melt passes, wherein the extruder housing is divided into two chambers by return screw threads disposed on the circumference of the worm, one of which chambers being provided with the intake opening and the other being provided with the exit opening, which chambers being interconnected by at least one channel in which the melt filter is disposed, and wherein each degasing opening is connected to the chamber provided with the exit opening and is disposed in axial distance from this exit opening, in which chamber the extruder worm has a geometry of its threads that causes a no-pressure-zone.
Apparatus of this kind are known (U.S. Pat. No. A 3,380,824). The melt discharged through the exit opening is supplied to a tool, for example to a mold, by which articles of thermoplastic synthetic plastics material are produced.
It is also known to subject synthetic plastics material scrap to a recycling process by supplying the synthetic plastics material scrap through an intake opening into the extruder housing and to melt it therein and to separate the impurities from the melt by means of the filter. Within this, there occurs the problem that on the one hand, the impurities contained in the synthetic plastics material scrap must be removed, but on the other hand the gases released during plasticizing must be removed, which gases, for example, are created by the fact that printing colours adhering on the scrap synthetic plastics material release gaseous components at a certain temperature or are even entirely transformed into the gaseous condition. For the removal of such gaseous components, a degasing opening is provided within the known construction, which opening--when seen in flow direction of the synthetic plastics material--is disposed immediately behind the screen constituting the filter. The purified and degased synthetic plastics material is then introduced into the second chamber and is conveyed through this towards the exit opening. Investigations have shown that such an apparatus does not meet the requirements. The investigations have shown that degasing of the treated synthetic plastics material is not sufficient, since the synthetic plastics material is subjected to turbulences when passing through the filter, whereby gaseous components which optionally have already been released from the synthetic plastics material or are ready for such a release, are worked in again into the synthetic plastics material mass.
In a one-part-worm, which, therefore, is disposed in one single chamber, it is also known to dispose a filter purifying the synthetic plastics material immediately before the exit opening. Within this, the degasing openings are disposed before the filter, so that the melt contains impurities also after degasing. In front of the filter or, respectively, of the exit opening, the extruder worm must have such a lead that the required conveying pressure is created in order to overcome the resistances caused by the filter and by the tool connected to the exit opening. Therefore, it depends from the geometry of the extruder worm in this section, which maximum resistance can be overcome. If this maximum resistance is exceeded, for example due to soiled filter screens, the output of the melt through the exit opening is decreased and there results a back pressure of the melt that causes an undesired exit of the melt through the degasing openings.
Other known apparatus show still further disadvantages. Thus, the use of fine-meshed filter screens as it is suitable for the production of final products of high quality, depends from the melt viscosity of the synthetic plastics material, from the entire filter surface and from the throughput, and is limited therefore. Further, within the known apparatus, gases may be produced by the impurities contained in the melt after this melt has passed the degasing openings, so that then a final product is obtained which is not completely degased. The same holds for inhomogenities in the melt which pass the degasing openings in a not molten condition and are molten only later on, so that they start to degase only then.
The present invention has at its object to avoid the disadvantages of the known apparatus and to provide an apparatus for the extrusion of thermoplastic synthetic plastics material in which no material leaves the degasing openings and in which all impurities and inhomogenities are filtered out already before the degasing operation, which apparatus, however, provides for a reliable degasing of the synthetic plastics material. The invention solves this task, starting from the initially described known apparatus, by the features, that each degasing opening--when seen in conveying direction of the synthetic plastics material--is disposed behind the junction of each channel and of the chamber provided with the exit opening, the no-pressure-zone being disposed between this junction and the degasing opening, and that the junction of each channel and of the chamber provided with the exit opening is disposed immediately adjacent the return screw threads. Degasing of the synthetic plastics material, therefore, occurs only at a time at which the synthetic plastics material, after having passed the melt filter, has been introduced again into the worm housing. From the moment of passing the melt filter till the moment at which the synthetic plastics material reaches the degasing opening or the degasing openings, the gases contained in the synthetic plastics material have sufficient time to escape from the synthetic plastics material, inasmuch during this time interval the synthetic plastics material is not subjected to a substantial pressure because during this time interval it is disposed within the no-pressure-zone of the extruder worm, which zone according to the invention--when seen in flow direction of the synthetic plastics material--is disposed before the degasing openings, however, within the initially described apparatus it is disposed behind the degasing opening.
To dispose the mouths by which each channel leads the filtered synthetic plastics material into the chamber leading to the exit opening immediately adjacent the return screw threads, ensures that synthetic plastics material is reliably supplied to the return screw threads. This advantage is not given within the initially described known construction, because there the opening through which the filtered material is introduced again into the worm housing, is displaced for about half a lead of the screw threads of the extruder screw. Therefore, the extruder screw tends to convey the material away from the return screw threads, so that the return screw threads cannot sufficiently be supplied with synthetic plastics material. This has as a consequence, that the two chambers are not reliably tightened relative to each other and there is the danger that the return screw threads run dry. Within the inventive construction, it is ensured that these disadvantages are avoided, and, in addition, the advantage is obtained that the leads of the return screw threads are always cleaned by already filtered material, so that in the region of the return screw threads no cracking of the synthetic plastics material can occur.
By disposing the degasing openings spaced apart from the synthetic plastics material filters or, respectively, by the thereby obtained degasing of the synthetic plastics material only at a substantially later moment as that at which the melt passes the melt filter, the advantage is obtained that in case of an eventual back pressure due to blocking of a melt filter, no material can escape through the degasing openings. By the fact that inhomogenities and impurities are removed from the melt before it reaches the degasing openings, it is also avoided that further gases are created after passing these degasing openings. Lastly, the inventive construction shows the advantage that the feed pressure on the melt in the region of the exit opening is reduced, since only the resistance of the tool must be overcome.
As it is well known, return screw threads are tightening screw threads having a small lead of opposite hand with respect to the worm threads. By these return screw threads a small amount, for example about 1 to 1.5%, of the melt is conveyed in return direction from the chamber provided with the exit opening and the degasing openings into the chamber provided with the intake opening. Thereby it is avoided in any case that synthetic plastics material containing impurities can reach the chamber provided with the exit opening and the degasing openings directly, by by-passing the melt filter.
According to a preferred embodiment of the invention also the inlet opening of each channel leading from the chamber provided with the intake opening to a melt filter, is disposed immediately adjacent the return screw threads. Thereby dead corners are avoided in which the synthetic plastics material may collect and may stay within the apparatus for a longer time so that the synthetic plastics material may be subjected to thermic deteriorations.
According to a preferred embodiment of the invention, the no-pressure-zone of the extruder worm extends up to the degasing openings, the extruder worm being provided in connection to this zone of no-pressure with a geometry of its threads that causes a discharging zone extending up to the exit opening. In such a manner, maximum time is given to the synthetic plastics material for the release in order to degas, and, on the other hand, it is ensured at the exit opening that the melt is fed there into the tool with the necessary pressure. Suitably, the no-pressure-zone immediately neighbours the mouth of the channel that leads into the chamber provided with the exit opening, in order to ensure a maximum length of the no-pressure-zone.
In order to make the apparatus suitable also for processing such kinds of synthetic plastics material which do not require a degasing operation, according to a further embodiment of the invention at least one degasing opening is adapted to be closed.
In order to avoid dead corners, it is suitable within the spirit of the invention, if the wall of the inlet opening or, respectively, of the mouth of the channel is flush with the front surface of that section of the extruder worm that is provided with the return screw threads.
In order to ensure a simple possibility for producing those sections of a channel which intersect the extruder housing, it is suitable within the spirit of the invention that those channel sections that are provided with the inlet openings or, respectively, with the mouths, extend radially in the extruder housing.